The requirement of keeping electromagnetic interferences (EMI) low and providing a sufficient electromagnetic compatibility (EMC) control circuits for controlling the switching operation of switching transistors (e.g., MOSFETs) are often designed to avoid abrupt changes of the transistor load current (i.e., the drain current in case of a MOSFET) as well as of the corresponding voltage drop (i.e., drain-source-voltage in case of a MOSFET) across the transistor. When switching the load current on and off the resulting load current gradient should not to be too steep so as to avoid high frequency signal components present in the resulting current waveform. Such a behavior of the control circuit and the respective control method is often referred to as “edge shaping.”
However, a slow and smooth switching of the load current results in higher switching losses which are generally undesired and thus a conflict of design goals exists. Higher switching losses usually lead to higher chip temperatures in the silicon body in which the semiconductor switch is integrated. As the electric behavior of a transistor is, in general, temperature dependent, an increasing chip temperature may, under certain operating conditions, eventually lead to even higher currents and correspondingly higher losses and thus to thermally unstable operation states. Current filamentation, the formation of “hot spots” within the semiconductor body, and a general degradation or even destruction of the semiconductor switch may be the result of such thermally unstable operating states.
In view of the above there is a need for a control circuit and a respective method for controlling the switching operation of a semiconductor switch which helps to avoid thermally unstable operation states of the semiconductor switch while keeping electromagnetic interferences (EMI) low during normal operation states.